1. Field of the Invention
The present invention relates to a nuclear magnetic resonance apparatus which uses a nuclear magnetic resonance (NMR) signal and which can be utilized in a medical field, and in analyzing a component and structure of an industrial material, agricultural produce, and the like. The present invention particularly relates to a nuclear magnetic resonance apparatus for generating a strong static magnetic field which is comparable to a conventional superconducting magnet in a homogeneity aria without using liquid helium.
2. Description of Related Art
A nuclear magnetic resonance is a phenomenon in a magnetic system including a magnetic moment and angular momentum, and is a resonance phenomenon in a frequency (Larmor frequency) inherent in the magnetic system. For example, as shown in FIG. 1, a static magnetic field H0 made by a magnet is applied to a sample, and a radio frequency magnetic field H1 is further applied to the sample from a direction vertical to the static magnetic field via a transmission coil. At present, a pulse nuclear magnetic resonance(NMR) apparatus is a mainstream, in which a very short (3 to 6 xcexcs) and strong high-frequency pulse is applied to the sample, and all signals spreading in a chemical shift are simultaneously resonated and simultaneously observed.
Moreover, in order to obtain a image, a magnetic field whose strength differs with a position called a gradient magnetic field is superimposed onto the static magnetic field, and a position is identified by shifting a resonance frequency for each position. An image method of exciting (selectively exciting) a predetermined section only by a necessary thickness with a high frequency, subsequently applying the gradient magnetic field in two directions in the section, and obtaining the sectional image by a two-dimensional Fourier method is generally used.
The aforementioned nuclear magnetic resonance apparatus (hereinafter referred to as an NMR apparatus) utilizing the aforementioned nuclear magnetic resonance phenomenon is basically constituted of a magnet for forming the static magnetic field, a coil for generating another high-frequency pulse and detecting an NMR signal, a receiver for receiving the NMR signal, and the like. Data useful in analyzing a structure of an organic compound, such as a chemical shift amount of each atom and spin-spin coupling constant can be obtained by the NMR apparatus.
Moreover, a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) utilizing the nuclear magnetic resonance phenomenon is constituted of: at least a magnet as static magnetic field generation means; a gradient magnetic field for applying space information to the signal; a high-frequency irradiation system; an NMR signal detection system; a probe coil which surrounds a test object such as a human body and actually performs high-frequency irradiation and signal detection; and a controller for controlling these components and processing the obtained signal. A space distribution of a nuclide which generates the signal is visualized by the nuclear magnetic resonance (NMR) signal obtained by irradiating the test object disposed in the presence of the static magnetic field with the high frequency. Since the MRI apparatus does not use a X-ray, the apparatus is safe not only for the human body subjected to a measuring operation but also for an object to be measured including the human body, a sufficient resolution is obtained, and a practical value is remarkably high.
As the static magnetic field generating magnet constituting the nuclear magnetic resonance apparatus, a resistive magnet of 0.5 to 2.2 T, and a superconducting magnet of 0.5 to 18.8 T have heretofore been used, and a permanent magnet is also used in some case. The static magnetic field generating magnet of the nuclear magnetic resonance apparatus has an enhanced sensitivity for a ferromagnetic field, and enables analysis of a large amount of detailed information. Therefore, the superconducting magnet using a superconducting material is superior in the strength, stability and uniformity of the magnetic field.
Therefore, in the recent nuclear magnetic resonance apparatus, the superconducting magnet using a superconducting coil formed of a metal-based superconducting wire material such as niobium and titanium is used to form a main magnetic field (static magnetic field). However, when the superconducting coil is utilized, liquid helium is used to cool the coil at an extremely low temperature. This raises a problem that a large amount of expensive liquid helium is required and running cost is high.
Moreover, the metal-based superconducting wire material such as niobium and titanium is produced by a complicated manufacturing process and thermal treatment. Therefore, the superconducting coil is much more expensive than a usual electromagnet coil formed of a copper wire, and the apparatus main body becomes extremely expensive. Additionally, a refrigerant (liquid helium and liquid nitrogen) utility for operating the superconducting magnet requires a special technique, and is technically complicated and intricate. Therefore, it is difficult to say that the utilization is a simple technique. These big problems hinder a high-performance nuclear magnetic resonance apparatus from spreading.
Furthermore, since the superconducting magnet requires a large cooling structure, and a leak magnetic field is also huge, an exclusive room for installing the magnet is necessary. This remarkably limits an apparatus installation condition, and also limits an apparatus utilization field.
On the other hand, an example of a small and simple nuclear magnetic resonance apparatus is proposed in Japanese Patent Application Laid-Open No. 135823/1997, in which a direct cooling type superconducting magnet is used instead of the conventional helium cooling type superconducting magnet. This nuclear magnetic resonance apparatus is more convenient than the apparatus using the conventional helium cooling type superconducting magnet, but the superconducting coil formed of the superconducting wire material is used to form the main magnetic field. Since the superconducting wire material is extremely expensive, the whole apparatus becomes expensive. Moreover, since a refrigerator is used to cool the superconducting coil in a vacuum container, a coil portion becomes large-sized. In this case, the advantage that the apparatus is small-sized and convenient cannot sufficiently be utilized. Furthermore, since a heat capacity of the superconducting coil is large, a time necessary for cooling the coil at a predetermined temperature with the refrigerator is long. There is also a problem that a time from the start of cooling until the start of measurement is long.
To solve the aforementioned conventional problem, the present applicant of the present invention has developed and filed a prior application for the nuclear magnetic resonance apparatus in which a high-temperature superconductor is used (Japanese Patent Application Laid-Open No. 248810/1999). In this apparatus, a superconducting current flows through the high-temperature superconductor which is cooled in a vacuum insulating container and to which the magnetic field is applied. Then, the superconductor captures the magnetic field to constitute a magnetic field supply member, the magnetic field is used as the main magnetic field, and the NMR signal of a material to be measured disposed in the magnetic field is detected by a detection coil and spectrometer disposed adjacent to the material to be measured.
In the nuclear magnetic resonance apparatus, the superconducting current flows through the high-temperature superconductor, the magnetic field is captured, and the magnetic field supply member is constituted. Therefore, a strong static magnetic field comparable to the conventional superconducting magnet can be formed without using the expensive liquid helium.
However, as shown in FIG. 2, a magnetic field distribution generated in the conventional high-temperature superconductor forms a mountain-shaped distribution whose center is high in strength. This raises a problem that a uniform magnetic field cannot be applied to the material (sample) to be measured. Moreover, for example, when another permanent magnet, electromagnetic soft iron, or the like is disposed in a position apart from the high-temperature superconductor in order to uniform the magnetic field, a uniform magnetic field area can be formed. However, there is a problem that the magnetic field strength is remarkably lowered.
The present invention has been developed to solve the problem. That is, an object of the present invention is to provide a nuclear magnetic resonance apparatus which can form a strong static magnetic field comparable to a conventional superconducting magnet without using a refrigerant (liquid helium) essential for operating the conventional superconducting magnet, and which can make homogeneity aria of the static magnetic field.
According to the present invention, there is provided a nuclear magnetic resonance apparatus comprising: a cup-shaped high-temperature superconductor (20) having a hollow cylindrical shape or a hollow cylindrical portion cooled at a temperature not more than a superconducting transition temperature in a vacuum insulating container (22); and a detection coil (12) for detecting an NMR signal of a material (11) to be measured inserted into the hollow cylindrical portion (20a) of the high-temperature superconductor. The high-temperature superconductor is magnetized in an axial direction, a static magnetic field is thereby generated in the hollow cylindrical portion in a cylinder axial direction, and the NMR signal of the material to be measured disposed in the magnetic field is detected by the detection coil and a spectrometer.
According to the constitution of the present invention, when a hole is made in a center of a bulk magnet calcined in a cylindrical shape in the axial direction, a uniform magnetic field distribution can be obtained inside the hole.
That is, when the center of the bulk magnet calcined in the cylindrical shape is holed in the axial direction, the high-temperature superconductor (20) having the hollow cylindrical shape can be formed. A superconducting current is generated centering on an axial center of the high-temperature superconductor to magnetize the high-temperature superconductor, so that the high-temperature superconductor can function similarly as the conventional superconducting coil. Therefore, a substantially uniform magnetic field distribution can be obtained in a state equivalent to a state in which the coil is wound.
Therefore, the strong static magnetic field comparable to the conventional superconducting magnet can be formed without using the refrigerant (liquid helium) essential for operating the conventional superconducting magnet, and the strength distribution of the static magnetic field can be homogeneous.
According to a preferred embodiment of the present invention, a plurality of high-temperature superconductors (20) are coaxially disposed opposite to each other in the axial direction, or at a gap therebetween.
According to the constitution, when the static magnetic field formed by the plurality of high-temperature superconductors (20) is multilayered, the strong static magnetic field can be formed in a broader area.
Moreover, the detection coil (12) is preferably disposed in the vacuum insulating container (22). In the constitution, the detection coil (12) is thermally shut off from the outside in the vacuum insulating container and cooled in the same temperature range as that of the high-temperature superconductor under insulation. The detection coil (12) reduces a thermal noise, and improves electric conductivity, and sensitivity is enhanced.
Furthermore, the high-temperature superconductor (20) is an oxide superconductor whose main component is represented by RExe2x80x94Baxe2x80x94Cuxe2x80x94O. The superconductor contains 0 to 50% of at least one of silver, platinum and cerium, and RE is synthesized by at least one of yttrium (element symbol: Y), samarium (Sm), lanthanum (La), neodymium (Nd), europium (Eu), gadolinium (Gd), erbium (Er), ytterbium (Yb), dysprosium (Dy), and holmium (Ho). Moreover, a structure is included in which a superconducting phase having a superconducting transition temperature of 90 K to 96 K in terms of an absolute temperature and an allotrope insulating phase disposed inside the superconducting phase are dispersed with a particle size of 50 xcexcm or less, preferably 10 xcexcm or less.
The yttrium-based, neodymium-based, samarium-based, and other high-temperature superconductors having the superconducting transition temperature Tc of 90 Kelvin (K) or more in terms of the absolute temperature are synthesized in a so-called melting method of heating and melting the materials once at a temperature higher than a melting point, and coagulating the materials again. Then, a molded material with a rough and large grown crystal is obtained, and this is called the superconducting bulk. A structure in which the insulating phase is finely dispersed is obtained in a superconducting parent phase, a pinned point attributed to the presence of the dispersed phase captures a magnetic flux, and the superconducting bulk serves as a pseudo permanent magnet.
The superconducting bulk synthesized by the melting method has the superconducting transition temperature of 90K to 96K in terms of the absolute temperature, and includes a structure in which a large crystal formed of the superconducting phase with a size of 1 mm to 100 mm, and the allotrope insulating phase disposed inside the large crystal are dispersed with a particle size of 50 xcexcm or less (preferably 10 xcexcm or less).
When the allotrope insulating phase dispersed in the structure is 50 xcexcm or less during synthesis of the high-temperature superconductor, a satisfactory magnetic field capture property appears, a 0.5 T class magnetic field is generated, and the high-temperature superconductor can be used in the present invention. In a certain system to which platinum and cerium are added, the insulating phase can be controlled to be 10 xcexcm or less. In this case, the magnetic field having an extremely excellent property of 1 T class or a higher class or several T can be captured, and the present invention can highly be utilized. Moreover, in a system to which 0 to 50% of silver is added, a crack easily introduced in a synthesis stage or a use stage is prevented from being generated or propagated, and a large-sized high-temperature superconductor can advantageously and soundly be synthesized and used.
When the high-temperature superconductor is used, a superconducting state can be realized at the absolute temperature of 90K or more which exceeds a boiling point of 77K of liquid nitrogen. Additionally, when the high-temperature superconductor is cooled at a lower temperature, the strong static magnetic field comparable to the conventional superconducting magnet can be formed.
The magnetic field of the high-temperature superconductor can be magnetized by the static magnetic field. When the strong static magnetic field is generated by the conventional metal-based superconducting coil, and the magnetic field is captured by the high-temperature superconductor, the high-temperature superconductor can be magnetized by the strong static magnetic field able to be generated by the metal-based superconducting coil.
Moreover, the high-temperature superconductor may also be magnetized by a pulse magnetic field. When a large magnetic field is momentarily generated in the coil to magnetize the high-temperature superconductor, the high-temperature superconductor can more easily be magnetized.
Furthermore, it is preferable to cool the high-temperature superconductor at a temperature not more than the superconducting transition temperature in a refrigerant, by a cooling portion of a refrigerator, or by the refrigerant cooled by the refrigerator. As the refrigerator, a pulse tube refrigerator, GM cycle refrigerator, Solvay cycle refrigerator, Stirling cycle refrigerator, and Peltier refrigerator are preferably used alone or as a combination of two or more thereof.
As the refrigerant, a gas, liquid, or solid (excluding helium) of nitrogen, oxygen, argon, helium, neon, hydrogen, or the like can be used. This can easily cool the high-temperature superconductor at a temperature which is not more than the superconducting transition temperature (e.g., about 90K).
In a structure for cooling the high-temperature superconductor, copper or another metal, or an alumina single crystal or another heat transfer material directly contacts and cools the high-temperature superconductor, and the metal or the heat transfer material is cooled by the refrigerants such as liquid nitrogen or the aforementioned various refrigerators.
In another structure, the high-temperature superconductor is cooled by the liquid, gas or solid refrigerant such as liquid nitrogen, liquid helium, and solid nitrogen gas which is cooled by the refrigerator. In this case, since the superconductor is separated from the refrigerator, a mechanical influence of the refrigerator can be cut off, and measurement precision can be enhanced.
Other objects and advantageous characteristics of the present invention will be apparent from the following description with reference to the accompanying drawings.